linux-kernel - Re: [PATCH] vfio/pci: Virtualize Maximum Payload Size

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Date:   Wed, 20 Sep 2017 16:26:25 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Sinan Kaya <okaya@...eaurora.org>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, jcm@...hat.com,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] vfio/pci: Virtualize Maximum Payload Size

Hi Sinan,

On 20/09/2017 15:01, Sinan Kaya wrote:
> On 9/20/2017 3:59 AM, Auger Eric wrote:
>>> My impression is that MRRS is predominantly device and driver
>>> dependent, not topology dependent.  A device can send a read request
>>> with a size larger than MPS, which implies that the device supplying
>>> the read data would split it into multiple TLPs based on MPS.
>> I read that too on the net. However in in 6.3.4.1. (3.0. Nov 10), Rules
>> for SW Configuration it is written:
>> "Software must set Max_Read_Request_Size of an isochronous-configured
>> device with a value that does not exceed the Max_Payload_Size set for
>> the device."
>>
>> But on the the other hand some drivers are setting the MMRS directly
>> without further checking the MPS?
> 
> We discussed this on LPC. MRRS and MPS are two independent concepts and
> are not related to each other under normal circumstances. 
> 
> The only valid criteria is that MRRS needs to be a multiple of MPS.
> 
> https://linuxplumbersconf.org/2017/ocw//system/presentations/4732/original/crs.pdf
> 
> Because completions are required to be a minimum of MPS size. If MRRS > MPS,
> read response is sent as multiple completions.

With that patch, you can end up with MRRS < MPS. Do I understand
correctly this is an issue?

Thanks

Eric
> 
> The only reason you want to match MRRS==MPS is that you don't want a single
> device to hog system resources. You can have MRRS 4k and MPS 128 bytes. Completions
> come in as 128 x N packets on the PCI bus. 
> 
> If you are sharing the same PCI bus with some other device, switch; you are effectively
> stalling other devices. That's why, isochronous devices are requesting small MRRS.
> 
> An Isochronous device is an exception not norm.
>